1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method by which a printing is performed by ejecting liquid droplets (hereinafter referred to as ink) to a printing medium for performing a printing. In particular, the present invention relates to a structure for performing, to a printing medium, a maintenance ejection (also called as preliminary ejection) for maintaining a favorable ejection performance of a printing head for ejecting ink.
2. Description of the Related Art
An ink jet printing method has an advantage in that low noise and low running cost are realized, the apparatus can have a smaller size, and the apparatus that performs color printing can be realized easily for example. Printing heads for ejecting color ink in particular have been involved with a tendency where a droplet size of ink droplets ejected from printing heads have been reduced from about 15 pl through 5 pl to 2 pl. As a result, ink droplets having a smaller dot size constituting a printed image can be provided so as to reduce the granularities of a half tone part in a gray scale image, and a halftone part and a highlight part in a color photograph image. With an advancement and widespread diffusion of digital devices in particular, printing of a high-resolution image and a high-quality printing, such as a photograph printing, have been increasingly required and a further smaller liquid droplet has been also required. In according with this, an ink ejection nozzle of a printing head has been provided with a smaller opening diameter.
When a nozzle has a smaller opening diameter, defective ejection due to ink having an increased viscosity or the like (e.g., an abnormal ejection in which ink is ejected in a bent direction or an ink droplet does not reach a paper, and an ejection failure in which ejection itself is not performed) is caused easily. In order to solve the defective ejection as described above, a conventional ink jet printing apparatus is structured so that a maintenance ejection (preliminary ejection) is performed just before a printing operation or in the middle of a printing operation with a fixed interval, in which ink is ejected to a predetermined portion of the apparatus (e.g., an ink receiver including a discharged ink absorber). Thus, ink having an increased viscosity in the vicinity of an opening of a nozzle or a nozzle flow path can be ejected to maintain a correct ink ejection performance. This maintenance ejection performs a few to a dozen of ink droplet ejection with an interval of about 2 to 15 seconds, depending on the ejection power of a used printing head, the dry level of used ink, or an environment temperature.
However, the maintenance ejection is performed during a printing operation to a printing medium or during supply or discharge operation of a plurality of pages of printing medium and thus requires a printing head to be moved to a predetermined position other than the position of a printing medium (e.g., discharged ink absorber). Thus, a tradeoff relation is established between a time required for the maintenance ejection and a time required for a unit of printing data (hereinafter referred to as print throughput). In a high speed printing mode in which a printing head is moved with a high speed or ink is ejected from a printing head with the maximum driving frequency in order to minimize the time required for a printing operation for each page in particular, the time loss due to the maintenance ejection operation required for the maintenance is relatively high. For example, this loss is so high as to occupy a few to a dozen of percentages of the entire printing time.
Specifically, this time loss is investigated for a case where all nozzles in a range of a width of a nozzle arrangement of a printing head are used to print to-be-printing data for one line by one scanning operation. When assuming that an A4-sized printing medium has a printable region of 8″×11″ and when the entirety of this printing medium is printed with the data by a printing head having an ink droplet size of 5 pl, 256 nozzles arranged with 1200 dpi, and an arrangement width of 0.21 inch, about 52 scanning (scanning with printing head+paper line feed) are required. It is assumed that the printing head has a driving frequency of 15 kHz and a scanning speed of 25 inch/second. In this case, if a line feed time of a conveyed paper and the time required for the rise and decay of the scanning operation of a printing head are 0.1 second, a printing time for one line is about 0.52 seconds and a time required for the printing of one A4-sized paper is about 27 seconds. When assuming that an interval between maintenance ejections is 5 seconds, a rate of a time required for the maintenance ejection to one page can be approximately calculated as shown below. The maintenance ejection is performed five times for one page. One maintenance ejection requires a printing head to be additionally moved by a distance corresponding to the distance for one scanning operation. As a result, the rate of a time required for the maintenance ejection to one page can be calculated as: maintenance ejection by 5 scanning/printing by 52 scanning=0.096 that is nearly equal to about 10% time loss.
Furthermore, when a printing head has a reduced nozzle opening diameter so that ejected liquid droplets can have a size of about 2 pl, an interval between maintenance ejections is reduced to about 2 seconds. In this case, maintenance ejection is performed within a page 13 times. In this case, the same approximate calculation results in: maintenance ejection by 13 scanning/printing by 52 scanning=0.25 that is nearly equal to about 25% time loss. Thus, a decline of the print throughput is 2.5 times higher than the case of 5 pl is caused.
In view of the above, Japanese Patent Application Laid-open No. H8-112904 has disclosed a structure for improving the decline of a print throughput due to the maintenance ejection by a printing head. This structure reduces operations required for the maintenance ejection by ejecting such ink onto a printing medium that is not for an image formation purpose.
However, the structure in which a printing medium is subjected to the maintenance ejection causes separate inks for the maintenance ejection to be ejected into an image to be printed and thus may cause a case where dots by the separate inks are so noticeable that the dots can be visually recognized. The noticeable dots of ink ejected for the maintenance ejection causes a problem where the resultant printed image has a lower quality.
The present inventors have examined a size (area) of dots and the shape of a dot that are visually recognized easily. Specifically, ink ejection from a printing head to a printing medium was performed with respect to different volumes of ink droplets, different colors of inks, and different driving conditions to examine the visibility of the resultant dots in a sensory manner. The term “visibility” herein means how much the dots are noticeable and is determined by allowing a person having binocular vision of about 1.0 to 1.5 to see a printing medium on which the dots are formed with a distance of about 20 cm between the person and the printing medium.
As a result, it was found that, with a smaller dot formed by ejected ink, the visibility of the dot on the printing medium is reduced (which means that the dot is more difficultly seen) as is obviously expected and the respective dot shapes and colors have different visibilities.